[Polleke]

Hallo all,

I have been discussing the following problem: if you make a 70% ethanol solution to disinfect a bench or laminar flow cabinat, should you then make it with demineralized water, double destilled or MQ water or jut with tap water?

Some argue that you should use the most pure form of water in terms of: less contaminated with "germs", however I think this is not a problem and its more about the possibility of getting precipitation reactions in your tap water due to too many salts still present (and that the presence of bacteria in tap water doesnt matter at all).

[Dan]

I find it unlikely that any life from tapwater will survive in 70% ethanol, otherwise this would not be a very effective disinfectant after all.

I don't see a particularly good reason to use purified water in a disinfectant solution for lab benches.

[Babcock_Hall]

I usually make this with deionized water, but I have never thought about it before.  Tap water is fine, for the reason that Dan gave.  If I felt that the ions in tap water were something I wanted to avoid having on the benchtop, I would just use deionized water.

[Polleke]

I usually make this with deionized water, but I have never thought about it before.  Tap water is fine, for the reason that Dan gave.  If I felt that the ions in tap water were something I wanted to avoid having on the benchtop, I would just use deionized water.

But then again: the ions , would they make such a big difference?
The only idea that I could find in favor of using dH2O is because of the ions in hard water could perhaps start to form lime in bottles or perhaps cause precipitation and this make it possible for bacteria to grow in it, but this seems far fetched.

So I find it weird that so many labs use more expensive water to prepare the 70% solution rather then just use the tap water.

[Dan]

Bacteria will not grow in 70% ethanol regardless of precipitates. You can make up 70% ethanol and dump in as much lime as you like, there will be no life.

[Polleke]

Bacteria will not grow in 70% ethanol regardless of precipitates. You can make up 70% ethanol and dump in as much lime as you like, there will be no life.

Then perhaps is because of the lime blocking the squirt bottles?
This is the only real reason I can find: the lime blocking the squirt bottles or making small limespots in glasware perhaps.

[explosci]

The chance of lime build up blocking a squirt bottle is astronomically slow.

I would say any of the water in your list would work just fine.  But tap water will be the cheapest.

[rjb]

Polleke,

Tap water is fine... If your lab is anything like ours, I suspect the tap water has fewer microbeasties in it than the MilliQ anyhow!

One thing I would watch at audit time (particularly as if I remember rightly you work in the UK) is that it is technically illegal to use duty free spirit (most grades of ethanol in scientific use) for cleaning purposes and your employer could be fined if this is found to be the case... As an alternative you could always use 70% IMS (Industrial Methylated Spirits); its good enough for decon in the NHS so it must be pretty effective and it doesn't upset the HMRC  

Cheers

R

[AndyG]

Actually I've never thought about using Tap water .I used to use distilled water.

[Polleke]

Actually I've never thought about using Tap water .I used to use distilled water.

That is just it: every lab is used to using distilled water and I just wondered why!
If I tell them you can use tap water too... they just laugh at me and think I am crazy.

I really dont understand where the idea came from that you do need to use distilled water to make 70% ethanol.

[billnotgatez]

I assume it is required to pass audit
or
eliminate possible objections when doing research and you need to make sure no outside factors are present


wow talk about reviving an old post from the dead

[Polleke]

I assume it is required to pass audit
or
eliminate possible objections when doing research and you need to make sure no outside factors are present


wow talk about reviving an old post from the dead

I can understand this if you would need the 70% ethanol to do (for example) a DNA extraction or something molecular..
I can get that, however 90% of the 70% ethanol spray bottles are just used to clean the bench where you work.. I dont see the point in that. Its more a waste of demiwater.


And about passing the audit, I dont know, I doubt it to be honest they would check how you make your ethanol.

[discodermolide]

Don't worry the QA people know how to check everything. There are even SOP's to tell you how to blow your nose.

[Polleke]

Don't worry the QA people know how to check everything. There are even SOP's to tell you how to blow your nose.

haha true.

But for me its just about the fact that people make it with demiwater because someone told them to do it like that and because everyone is doing it like that.. they keep doing it like that.
Nobody seems to question it.

[Arkcon]

I don't think its unreasonable to specify distilled or deionized water for ethanol-based disinfecting solutions.  Tap water is an undefined product, it may contain random dissolved salts, depending on the time of the year, or the location.  Above, someone has stated that it might be more microbiologically clean that the DI system, but even if that is true sometimes, its not under the facilities control, so if its important, you never know if its always the case, for all times at all facilities.  You're not going to be allowed to use tap water, if there is an auditing process, it just looks sloppy.  And being slightly sloppy, when it doesn't really matter, doesn't make you look ingenious, it makes people wonder, "What other corners do they cut, 'round here."  c.f. : http://www.chemicalforums.com/index.php?topic=29560.msg112551#msg112551